Absence seizures afflict a significant percentage of children and up to 20% of all patients with epilepsy. Attempts to understand the molecular mechanisms of absence seizures may be facilitated by the use of lethargic (1h/1h) mutant mice, which harbor a single-locus defect and express absence seizures. GABAB receptor activation appears to be required for absence seizures in this model. The goal of this proposal is threefold. First, we will establish the role of GABAB receptors by examining the pharmacologic specificity with which they regulate absence seizures in 1h/1h mice. Second, we will identify the neural network in which GABAB receptors regulate these seizures. Third, we will use tissues from this neural network to characterize the mechanisms triggered by GABAB receptors in 1h/1h mice. In the short-term, the results forthcoming may establish GABAB antagonists as new antiabsence anticonvulsants in humans. In the long- term, these studies will help to characterize the aberrant genetic regulation of the mechanisms underlying absence seizures. This type of information may ultimately enable the control of absence seizures at the regulatory level. Specific aim 1 is to use EEG recordings to firmly establish the role of specific GABAB receptors and the role of specific neuronal structures in these seizures. The effects on seizure frequency of diverse compounds acting at GABAB and other receptors will be recorded by electrodes implanted in neocortex and subcortical sites. Specific aim 2 is to use radioligand binding techniques within slid- mounted sections and membranes of selected neuronal structures to characterize alterations intrinsic to the GABAB receptor in 1h/1h mice. These studies will begin to characterize mechanisms underlying the role of GABAB receptors in absence seizures. Specific aim 3 is to use microinjection techniques to test hypotheses that neuronal structures which express seizures (aim 1) or have enriched GABAB receptor binding (aim 2) regulate absence seizures in 1h/1h mice. This will be accomplished by measuring seizure frequency after microinjections of GABAB agonists or antagonists into candidate neuronal structures. These experiments will be used to choose appropriate tissues for the next aims. Specific aim 4 is to use biochemical techniques to test hypotheses that: i) GABAB receptor-mediated inhibition of net 45Ca+2 uptake into synaptosomes is greater in 1h/1h than nonepileptic control (+/+) mice; ii) GABAB inhibition of adenylate cyclase activity is greater in 1h/1h than +/+ mice. These experiments will help identify second messenger systems underlying the effect of GABAB receptors in absence seizures. Specific aim 5 is to use current- and voltage-clamp recording techniques in thalamic neurons of brain slices from 1h/1h and control mice to test hypotheses that: i) GABAB receptor-mediated IPSPs and low-threshold calcium spikes (LTCSs) are of greater magnitude in 1h/1h mice; ii) GABAB receptors are required for LTCSs. These experiments will further pinpoint candidate mechanisms that link GABAB receptor activation to absence seizures.